The field of the invention relates generally to a combination aircraft engine starter and aircraft electrical power generator, and more specifically, to an induction machine based sensorless aircraft engine starter/electrical power generator.
Typically, aircraft include a machine for use in starting an aircraft engine as well as for use in generating electrical power once the aircraft engine is operating. The electric machine is configured to receive externally supplied electrical power to drive the machine, which in turn, starts the aircraft engine. For example, the electric machine is used to begin the gas turbine cycle, and once the cycle is started, the aircraft engine is operating. Once the aircraft engine is operating, the aircraft engine is configured to power the electric machine, which facilitates electrical power generation.
Known aircraft engine starters include a direct current (DC) machine that includes brushes. An advantage of the brushed DC machine is a capability of providing both high torque for engine starting and high power for power generation at speeds two to three times the engine idle speed. However, disadvantages of the brushed DC machine are high maintenance and low life-span for some applications. Maintenance involves regularly replacing the brushes and springs which carry the electric current, as well as cleaning or replacing the commutator.
At least some known brushless methods to overcome the shortfalls mentioned above have been tried. One method is a permanent magnet brushless machine method, but the performance is rather disappointing due to an inability to accomplish both high torque at starting and high power generation over a three to one speed range with the competitive weight and size compared with those of a brushed machine. A second known method is a switched reluctance machine based approach. The performance of the switched reluctance machine is also unsatisfactory due to the same reason as the permanent magnet brushless machine approach. The third method is an induction machine based approach, which because of the method of implementation, the method did not achieve satisfactory performance due to issues that were are not addressed appropriately, thus previous induction machine based approaches suffer the same problem as the permanent magnet brushless machine and the switched reluctance machine. A fourth method is a synchronous wound field machine approach, which has had considerable success in achieving the two critical criteria of high starting torque capability and high power generation capability with a single machine. However the cost of the machine prohibits the method to be readily applied in many cost competitive aerospace applications, particularly for engines on business jets. Moreover, the synchronous wound field machine includes a rotating electronic assembly, the rotating rectifier in the shaft and has to have another machine, the exciter, which increases parts requirements and complexity that significantly reduces the reliability of the system. In addition, these methods also introduce a mechanical position sensor, which further reduces the reliability and further increases the cost of the engine.